/*
 * Internal header for libusb
 * Copyright (C) 2007-2009 Daniel Drake <dsd@gentoo.org>
 * Copyright (c) 2001 Johannes Erdfelt <johannes@erdfelt.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#ifndef LIBUSBI_H
#define LIBUSBI_H

#include <config.h>

#include <stddef.h>
#include <stdint.h>
#include <time.h>
#include <stdarg.h>
#ifdef HAVE_POLL_H
#include <poll.h>
#endif

#include <libusb.h>
#include <version.h>

/* Inside the libusb code, mark all public functions as follows:
 *   return_type API_EXPORTED function_name(params) { ... }
 * But if the function returns a pointer, mark it as follows:
 *   DEFAULT_VISIBILITY return_type * LIBUSB_CALL function_name(params) { ... }
 * In the libusb public header, mark all declarations as:
 *   return_type LIBUSB_CALL function_name(params);
 */
#define API_EXPORTED LIBUSB_CALL DEFAULT_VISIBILITY

#define DEVICE_DESC_LENGTH        18

#define USB_MAXENDPOINTS    32
#define USB_MAXINTERFACES    32
#define USB_MAXCONFIG        8

struct list_head {
    struct list_head *prev, *next;
};

/* Get an entry from the list
 *     ptr - the address of this list_head element in "type"
 *     type - the data type that contains "member"
 *     member - the list_head element in "type"
 */
#define list_entry(ptr, type, member) \
    ((type *)((uintptr_t)(ptr) - (uintptr_t)(&((type *)0L)->member)))

/* Get each entry from a list
 *    pos - A structure pointer has a "member" element
 *    head - list head
 *    member - the list_head element in "pos"
 *    type - the type of the first parameter
 */
#define list_for_each_entry(pos, head, member, type)            \
    for (pos = list_entry((head)->next, type, member);            \
         &pos->member != (head);                                \
         pos = list_entry(pos->member.next, type, member))

#define list_for_each_entry_safe(pos, n, head, member, type)    \
    for (pos = list_entry((head)->next, type, member),            \
         n = list_entry(pos->member.next, type, member);        \
         &pos->member != (head);                                \
         pos = n, n = list_entry(n->member.next, type, member))

#define list_empty(entry) ((entry)->next == (entry))

static inline void list_init(struct list_head *entry)
{
    entry->prev = entry->next = entry;
}

static inline void list_add(struct list_head *entry, struct list_head *head)
{
    entry->next = head->next;
    entry->prev = head;

    head->next->prev = entry;
    head->next = entry;
}

static inline void list_add_tail(struct list_head *entry,
    struct list_head *head)
{
    entry->next = head;
    entry->prev = head->prev;

    head->prev->next = entry;
    head->prev = entry;
}

static inline void list_del(struct list_head *entry)
{
    entry->next->prev = entry->prev;
    entry->prev->next = entry->next;
}

#define container_of(ptr, type, member) ({                      \
        const typeof( ((type *)0)->member ) *mptr = (ptr);    \
        (type *)( (char *)mptr - offsetof(type,member) );})

#define MIN(a, b)    ((a) < (b) ? (a) : (b))
#define MAX(a, b)    ((a) > (b) ? (a) : (b))

#define TIMESPEC_IS_SET(ts) ((ts)->tv_sec != 0 || (ts)->tv_nsec != 0)

enum usbi_log_level {
    LOG_LEVEL_DEBUG,
    LOG_LEVEL_INFO,
    LOG_LEVEL_WARNING,
    LOG_LEVEL_ERROR,
};

void usbi_log(struct libusb_context *ctx, enum usbi_log_level level,
    const char *function, const char *format, ...);

void usbi_log_v(struct libusb_context *ctx, enum usbi_log_level level,
    const char *function, const char *format, va_list args);

#if !defined(_MSC_VER) || _MSC_VER >= 1400

#ifdef ENABLE_LOGGING
#define _usbi_log(ctx, level, ...) usbi_log(ctx, level, __FUNCTION__, __VA_ARGS__)
#else
#define _usbi_log(ctx, level, ...) do { (void)(ctx); } while(0)
#endif

#ifdef ENABLE_DEBUG_LOGGING
#define usbi_dbg(...) _usbi_log(NULL, LOG_LEVEL_DEBUG, __VA_ARGS__)
#else
#define usbi_dbg(...) do {} while(0)
#endif

#define usbi_info(ctx, ...) _usbi_log(ctx, LOG_LEVEL_INFO, __VA_ARGS__)
#define usbi_warn(ctx, ...) _usbi_log(ctx, LOG_LEVEL_WARNING, __VA_ARGS__)
#define usbi_err(ctx, ...) _usbi_log(ctx, LOG_LEVEL_ERROR, __VA_ARGS__)

#else /* !defined(_MSC_VER) || _MSC_VER >= 1400 */

/* Old MS compilers don't support variadic macros. The code is simple, so we
 * repeat it for each loglevel. Note that the debug case is special.
 *
 * Support for variadic macros was introduced in Visual C++ 2005.
 * http://msdn.microsoft.com/en-us/library/ms177415%28v=VS.80%29.aspx
 */

static inline void usbi_info(struct libusb_context *ctx, const char *fmt, ...)
{
#ifdef ENABLE_LOGGING
    va_list args;
    va_start(args, fmt);
    usbi_log_v(ctx, LOG_LEVEL_INFO, "", fmt, args);
    va_end(args);
#else
    (void)ctx;
#endif
}

static inline void usbi_warn(struct libusb_context *ctx, const char *fmt, ...)
{
#ifdef ENABLE_LOGGING
    va_list args;
    va_start(args, fmt);
    usbi_log_v(ctx, LOG_LEVEL_WARNING, "", fmt, args);
    va_end(args);
#else
    (void)ctx;
#endif
}

static inline void usbi_err(struct libusb_context *ctx, const char *fmt, ...)
{
#ifdef ENABLE_LOGGING
    va_list args;
    va_start(args, fmt);
    usbi_log_v(ctx, LOG_LEVEL_ERROR, "", fmt, args);
    va_end(args);
#else
    (void)ctx;
#endif
}

static inline void usbi_dbg(const char *fmt, ...)
{
#ifdef ENABLE_DEBUG_LOGGING
    va_list args;
    va_start(args, fmt);
    usbi_log_v(NULL, LOG_LEVEL_DEBUG, "", fmt, args);
    va_end(args);
#else
    (void)fmt;
#endif
}

#endif /* !defined(_MSC_VER) || _MSC_VER >= 1400 */

#define USBI_GET_CONTEXT(ctx) if (!(ctx)) (ctx) = usbi_default_context
#define DEVICE_CTX(dev) ((dev)->ctx)
#define HANDLE_CTX(handle) (DEVICE_CTX((handle)->dev))
#define TRANSFER_CTX(transfer) (HANDLE_CTX((transfer)->dev_handle))
#define ITRANSFER_CTX(transfer) \
    (TRANSFER_CTX(USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)))

#define IS_EPIN(ep) (0 != ((ep) & LIBUSB_ENDPOINT_IN))
#define IS_EPOUT(ep) (!IS_EPIN(ep))
#define IS_XFERIN(xfer) (0 != ((xfer)->endpoint & LIBUSB_ENDPOINT_IN))
#define IS_XFEROUT(xfer) (!IS_XFERIN(xfer))

/* Internal abstractions for thread synchronization and poll */
#if defined(THREADS_POSIX)
#include <os/threads_posix.h>
#elif defined(OS_WINDOWS)
#include <os/threads_windows.h>
#endif

#if defined(OS_LINUX) || defined(OS_DARWIN) || defined(OS_OPENBSD)
#include <unistd.h>
#include <os/poll_posix.h>
#elif defined(OS_WINDOWS)
#include <os/poll_windows.h>
#endif

#if defined(OS_WINDOWS) && !defined(__GCC__)
#undef HAVE_GETTIMEOFDAY
int usbi_gettimeofday(struct timeval *tp, void *tzp);
#define LIBUSB_GETTIMEOFDAY_WIN32
#define HAVE_USBI_GETTIMEOFDAY
#else
#ifdef HAVE_GETTIMEOFDAY
#define usbi_gettimeofday(tv, tz) gettimeofday((tv), (tz))
#define HAVE_USBI_GETTIMEOFDAY
#endif
#endif

extern struct libusb_context *usbi_default_context;

struct libusb_context {
    int debug;
    int debug_fixed;

    /* internal control pipe, used for interrupting event handling when
     * something needs to modify poll fds. */
    int ctrl_pipe[2];

    struct list_head usb_devs;
    usbi_mutex_t usb_devs_lock;

    /* A list of open handles. Backends are free to traverse this if required.
     */
    struct list_head open_devs;
    usbi_mutex_t open_devs_lock;

    /* this is a list of in-flight transfer handles, sorted by timeout
     * expiration. URBs to timeout the soonest are placed at the beginning of
     * the list, URBs that will time out later are placed after, and urbs with
     * infinite timeout are always placed at the very end. */
    struct list_head flying_transfers;
    usbi_mutex_t flying_transfers_lock;

    /* list of poll fds */
    struct list_head pollfds;
    usbi_mutex_t pollfds_lock;

    /* a counter that is set when we want to interrupt event handling, in order
     * to modify the poll fd set. and a lock to protect it. */
    unsigned int pollfd_modify;
    usbi_mutex_t pollfd_modify_lock;

    /* user callbacks for pollfd changes */
    libusb_pollfd_added_cb fd_added_cb;
    libusb_pollfd_removed_cb fd_removed_cb;
    void *fd_cb_user_data;

    /* ensures that only one thread is handling events at any one time */
    usbi_mutex_t events_lock;

    /* used to see if there is an active thread doing event handling */
    int event_handler_active;

    /* used to wait for event completion in threads other than the one that is
     * event handling */
    usbi_mutex_t event_waiters_lock;
    usbi_cond_t event_waiters_cond;

#ifdef USBI_TIMERFD_AVAILABLE
    /* used for timeout handling, if supported by OS.
     * this timerfd is maintained to trigger on the next pending timeout */
    int timerfd;
#endif
};

#ifdef USBI_TIMERFD_AVAILABLE
#define usbi_using_timerfd(ctx) ((ctx)->timerfd >= 0)
#else
#define usbi_using_timerfd(ctx) (0)
#endif

struct libusb_device {
    /* lock protects refcnt, everything else is finalized at initialization
     * time */
    usbi_mutex_t lock;
    int refcnt;

    struct libusb_context *ctx;

    uint8_t bus_number;
    uint8_t device_address;
    uint8_t num_configurations;
    enum libusb_speed speed;

    struct list_head list;
    unsigned long session_data;
    unsigned char os_priv[0];
};

struct libusb_device_handle {
    /* lock protects claimed_interfaces */
    usbi_mutex_t lock;
    unsigned long claimed_interfaces;

    struct list_head list;
    struct libusb_device *dev;
    unsigned char os_priv[0];
};

enum {
  USBI_CLOCK_MONOTONIC,
  USBI_CLOCK_REALTIME
};

/* in-memory transfer layout:
 *
 * 1. struct usbi_transfer
 * 2. struct libusb_transfer (which includes iso packets) [variable size]
 * 3. os private data [variable size]
 *
 * from a libusb_transfer, you can get the usbi_transfer by rewinding the
 * appropriate number of bytes.
 * the usbi_transfer includes the number of allocated packets, so you can
 * determine the size of the transfer and hence the start and length of the
 * OS-private data.
 */

struct usbi_transfer {
    int num_iso_packets;
    struct list_head list;
    struct timeval timeout;
    int transferred;
    uint8_t flags;

    /* this lock is held during libusb_submit_transfer() and
     * libusb_cancel_transfer() (allowing the OS backend to prevent duplicate
     * cancellation, submission-during-cancellation, etc). the OS backend
     * should also take this lock in the handle_events path, to prevent the user
     * cancelling the transfer from another thread while you are processing
     * its completion (presumably there would be races within your OS backend
     * if this were possible). */
    usbi_mutex_t lock;
};

enum usbi_transfer_flags {
    /* The transfer has timed out */
    USBI_TRANSFER_TIMED_OUT = 1 << 0,

    /* Set by backend submit_transfer() if the OS handles timeout */
    USBI_TRANSFER_OS_HANDLES_TIMEOUT = 1 << 1,

    /* Cancellation was requested via libusb_cancel_transfer() */
    USBI_TRANSFER_CANCELLING = 1 << 2,

    /* Operation on the transfer failed because the device disappeared */
    USBI_TRANSFER_DEVICE_DISAPPEARED = 1 << 3,
};

#define USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer) \
    ((struct libusb_transfer *)(((unsigned char *)(transfer)) \
        + sizeof(struct usbi_transfer)))
#define LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer) \
    ((struct usbi_transfer *)(((unsigned char *)(transfer)) \
        - sizeof(struct usbi_transfer)))

static inline void *usbi_transfer_get_os_priv(struct usbi_transfer *transfer)
{
    return ((unsigned char *)transfer) + sizeof(struct usbi_transfer)
        + sizeof(struct libusb_transfer)
        + (transfer->num_iso_packets
            * sizeof(struct libusb_iso_packet_descriptor));
}

/* bus structures */

/* All standard descriptors have these 2 fields in common */
struct usb_descriptor_header {
    uint8_t  bLength;
    uint8_t  bDescriptorType;
};

/* shared data and functions */

int usbi_io_init(struct libusb_context *ctx);
void usbi_io_exit(struct libusb_context *ctx);

struct libusb_device *usbi_alloc_device(struct libusb_context *ctx,
    unsigned long session_id);
struct libusb_device *usbi_get_device_by_session_id(struct libusb_context *ctx,
    unsigned long session_id);
int usbi_sanitize_device(struct libusb_device *dev);
void usbi_handle_disconnect(struct libusb_device_handle *handle);

int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
    enum libusb_transfer_status status);
int usbi_handle_transfer_cancellation(struct usbi_transfer *transfer);

int usbi_parse_descriptor(unsigned char *source, const char *descriptor,
    void *dest, int host_endian);
int usbi_get_config_index_by_value(struct libusb_device *dev,
    uint8_t bConfigurationValue, int *idx);

/* polling */

struct usbi_pollfd {
    /* must come first */
    struct libusb_pollfd pollfd;

    struct list_head list;
};

int usbi_add_pollfd(struct libusb_context *ctx, int fd, short events);
void usbi_remove_pollfd(struct libusb_context *ctx, int fd);
void usbi_fd_notification(struct libusb_context *ctx);

/* device discovery */

/* we traverse usbfs without knowing how many devices we are going to find.
 * so we create this discovered_devs model which is similar to a linked-list
 * which grows when required. it can be freed once discovery has completed,
 * eliminating the need for a list node in the libusb_device structure
 * itself. */
struct discovered_devs {
    size_t len;
    size_t capacity;
    struct libusb_device *devices[0];
};

struct discovered_devs *discovered_devs_append(
    struct discovered_devs *discdevs, struct libusb_device *dev);

/* OS abstraction */

/* This is the interface that OS backends need to implement.
 * All fields are mandatory, except ones explicitly noted as optional. */
struct usbi_os_backend {
    /* A human-readable name for your backend, e.g. "Linux usbfs" */
    const char *name;

    /* Perform initialization of your backend. You might use this function
     * to determine specific capabilities of the system, allocate required
     * data structures for later, etc.
     *
     * This function is called when a libusb user initializes the library
     * prior to use.
     *
     * Return 0 on success, or a LIBUSB_ERROR code on failure.
     */
    int (*init)(struct libusb_context *ctx);

    /* Deinitialization. Optional. This function should destroy anything
     * that was set up by init.
     *
     * This function is called when the user deinitializes the library.
     */
    void (*exit)(void);

    /* Enumerate all the USB devices on the system, returning them in a list
     * of discovered devices.
     *
     * Your implementation should enumerate all devices on the system,
     * regardless of whether they have been seen before or not.
     *
     * When you have found a device, compute a session ID for it. The session
     * ID should uniquely represent that particular device for that particular
     * connection session since boot (i.e. if you disconnect and reconnect a
     * device immediately after, it should be assigned a different session ID).
     * If your OS cannot provide a unique session ID as described above,
     * presenting a session ID of (bus_number << 8 | device_address) should
     * be sufficient. Bus numbers and device addresses wrap and get reused,
     * but that is an unlikely case.
     *
     * After computing a session ID for a device, call
     * usbi_get_device_by_session_id(). This function checks if libusb already
     * knows about the device, and if so, it provides you with a libusb_device
     * structure for it.
     *
     * If usbi_get_device_by_session_id() returns NULL, it is time to allocate
     * a new device structure for the device. Call usbi_alloc_device() to
     * obtain a new libusb_device structure with reference count 1. Populate
     * the bus_number and device_address attributes of the new device, and
     * perform any other internal backend initialization you need to do. At
     * this point, you should be ready to provide device descriptors and so
     * on through the get_*_descriptor functions. Finally, call
     * usbi_sanitize_device() to perform some final sanity checks on the
     * device. Assuming all of the above succeeded, we can now continue.
     * If any of the above failed, remember to unreference the device that
     * was returned by usbi_alloc_device().
     *
     * At this stage we have a populated libusb_device structure (either one
     * that was found earlier, or one that we have just allocated and
     * populated). This can now be added to the discovered devices list
     * using discovered_devs_append(). Note that discovered_devs_append()
     * may reallocate the list, returning a new location for it, and also
     * note that reallocation can fail. Your backend should handle these
     * error conditions appropriately.
     *
     * This function should not generate any bus I/O and should not block.
     * If I/O is required (e.g. reading the active configuration value), it is
     * OK to ignore these suggestions :)
     *
     * This function is executed when the user wishes to retrieve a list
     * of USB devices connected to the system.
     *
     * Return 0 on success, or a LIBUSB_ERROR code on failure.
     */
    int (*get_device_list)(struct libusb_context *ctx,
        struct discovered_devs **discdevs);

    /* Open a device for I/O and other USB operations. The device handle
     * is preallocated for you, you can retrieve the device in question
     * through handle->dev.
     *
     * Your backend should allocate any internal resources required for I/O
     * and other operations so that those operations can happen (hopefully)
     * without hiccup. This is also a good place to inform libusb that it
     * should monitor certain file descriptors related to this device -
     * see the usbi_add_pollfd() function.
     *
     * This function should not generate any bus I/O and should not block.
     *
     * This function is called when the user attempts to obtain a device
     * handle for a device.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_ACCESS if the user has insufficient permissions
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since
     *   discovery
     * - another LIBUSB_ERROR code on other failure
     *
     * Do not worry about freeing the handle on failed open, the upper layers
     * do this for you.
     */
    int (*open)(struct libusb_device_handle *handle);

    /* Close a device such that the handle cannot be used again. Your backend
     * should destroy any resources that were allocated in the open path.
     * This may also be a good place to call usbi_remove_pollfd() to inform
     * libusb of any file descriptors associated with this device that should
     * no longer be monitored.
     *
     * This function is called when the user closes a device handle.
     */
    void (*close)(struct libusb_device_handle *handle);

    /* Retrieve the device descriptor from a device.
     *
     * The descriptor should be retrieved from memory, NOT via bus I/O to the
     * device. This means that you may have to cache it in a private structure
     * during get_device_list enumeration. Alternatively, you may be able
     * to retrieve it from a kernel interface (some Linux setups can do this)
     * still without generating bus I/O.
     *
     * This function is expected to write DEVICE_DESC_LENGTH (18) bytes into
     * buffer, which is guaranteed to be big enough.
     *
     * This function is called when sanity-checking a device before adding
     * it to the list of discovered devices, and also when the user requests
     * to read the device descriptor.
     *
     * This function is expected to return the descriptor in bus-endian format
     * (LE). If it returns the multi-byte values in host-endian format,
     * set the host_endian output parameter to "1".
     *
     * Return 0 on success or a LIBUSB_ERROR code on failure.
     */
    int (*get_device_descriptor)(struct libusb_device *device,
        unsigned char *buffer, int *host_endian);

    /* Get the ACTIVE configuration descriptor for a device.
     *
     * The descriptor should be retrieved from memory, NOT via bus I/O to the
     * device. This means that you may have to cache it in a private structure
     * during get_device_list enumeration. You may also have to keep track
     * of which configuration is active when the user changes it.
     *
     * This function is expected to write len bytes of data into buffer, which
     * is guaranteed to be big enough. If you can only do a partial write,
     * return an error code.
     *
     * This function is expected to return the descriptor in bus-endian format
     * (LE). If it returns the multi-byte values in host-endian format,
     * set the host_endian output parameter to "1".
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if the device is in unconfigured state
     * - another LIBUSB_ERROR code on other failure
     */
    int (*get_active_config_descriptor)(struct libusb_device *device,
        unsigned char *buffer, size_t len, int *host_endian);

    /* Get a specific configuration descriptor for a device.
     *
     * The descriptor should be retrieved from memory, NOT via bus I/O to the
     * device. This means that you may have to cache it in a private structure
     * during get_device_list enumeration.
     *
     * The requested descriptor is expressed as a zero-based index (i.e. 0
     * indicates that we are requesting the first descriptor). The index does
     * not (necessarily) equal the bConfigurationValue of the configuration
     * being requested.
     *
     * This function is expected to write len bytes of data into buffer, which
     * is guaranteed to be big enough. If you can only do a partial write,
     * return an error code.
     *
     * This function is expected to return the descriptor in bus-endian format
     * (LE). If it returns the multi-byte values in host-endian format,
     * set the host_endian output parameter to "1".
     *
     * Return 0 on success or a LIBUSB_ERROR code on failure.
     */
    int (*get_config_descriptor)(struct libusb_device *device,
        uint8_t config_index, unsigned char *buffer, size_t len,
        int *host_endian);

    /* Get the bConfigurationValue for the active configuration for a device.
     * Optional. This should only be implemented if you can retrieve it from
     * cache (don't generate I/O).
     *
     * If you cannot retrieve this from cache, either do not implement this
     * function, or return LIBUSB_ERROR_NOT_SUPPORTED. This will cause
     * libusb to retrieve the information through a standard control transfer.
     *
     * This function must be non-blocking.
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - LIBUSB_ERROR_NOT_SUPPORTED if the value cannot be retrieved without
     *   blocking
     * - another LIBUSB_ERROR code on other failure.
     */
    int (*get_configuration)(struct libusb_device_handle *handle, int *config);

    /* Set the active configuration for a device.
     *
     * A configuration value of -1 should put the device in unconfigured state.
     *
     * This function can block.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if the configuration does not exist
     * - LIBUSB_ERROR_BUSY if interfaces are currently claimed (and hence
     *   configuration cannot be changed)
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure.
     */
    int (*set_configuration)(struct libusb_device_handle *handle, int config);

    /* Claim an interface. When claimed, the application can then perform
     * I/O to an interface's endpoints.
     *
     * This function should not generate any bus I/O and should not block.
     * Interface claiming is a logical operation that simply ensures that
     * no other drivers/applications are using the interface, and after
     * claiming, no other drivers/applicatiosn can use the interface because
     * we now "own" it.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if the interface does not exist
     * - LIBUSB_ERROR_BUSY if the interface is in use by another driver/app
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*claim_interface)(struct libusb_device_handle *handle, int interface_number);

    /* Release a previously claimed interface.
     *
     * This function should also generate a SET_INTERFACE control request,
     * resetting the alternate setting of that interface to 0. It's OK for
     * this function to block as a result.
     *
     * You will only ever be asked to release an interface which was
     * successfully claimed earlier.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*release_interface)(struct libusb_device_handle *handle, int interface_number);

    /* Set the alternate setting for an interface.
     *
     * You will only ever be asked to set the alternate setting for an
     * interface which was successfully claimed earlier.
     *
     * It's OK for this function to block.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if the alternate setting does not exist
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*set_interface_altsetting)(struct libusb_device_handle *handle,
        int interface_number, int altsetting);

    /* Clear a halt/stall condition on an endpoint.
     *
     * It's OK for this function to block.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if the endpoint does not exist
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*clear_halt)(struct libusb_device_handle *handle,
        unsigned char endpoint);

    /* Perform a USB port reset to reinitialize a device.
     *
     * If possible, the handle should still be usable after the reset
     * completes, assuming that the device descriptors did not change during
     * reset and all previous interface state can be restored.
     *
     * If something changes, or you cannot easily locate/verify the resetted
     * device, return LIBUSB_ERROR_NOT_FOUND. This prompts the application
     * to close the old handle and re-enumerate the device.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if re-enumeration is required, or if the device
     *   has been disconnected since it was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*reset_device)(struct libusb_device_handle *handle);

    /* Determine if a kernel driver is active on an interface. Optional.
     *
     * The presence of a kernel driver on an interface indicates that any
     * calls to claim_interface would fail with the LIBUSB_ERROR_BUSY code.
     *
     * Return:
     * - 0 if no driver is active
     * - 1 if a driver is active
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*kernel_driver_active)(struct libusb_device_handle *handle,
        int interface_number);

    /* Detach a kernel driver from an interface. Optional.
     *
     * After detaching a kernel driver, the interface should be available
     * for claim.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
     * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - another LIBUSB_ERROR code on other failure
     */
    int (*detach_kernel_driver)(struct libusb_device_handle *handle,
        int interface_number);

    /* Attach a kernel driver to an interface. Optional.
     *
     * Reattach a kernel driver to the device.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NOT_FOUND if no kernel driver was active
     * - LIBUSB_ERROR_INVALID_PARAM if the interface does not exist
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
     *   was opened
     * - LIBUSB_ERROR_BUSY if a program or driver has claimed the interface,
     *   preventing reattachment
     * - another LIBUSB_ERROR code on other failure
     */
    int (*attach_kernel_driver)(struct libusb_device_handle *handle,
        int interface_number);

    /* Destroy a device. Optional.
     *
     * This function is called when the last reference to a device is
     * destroyed. It should free any resources allocated in the get_device_list
     * path.
     */
    void (*destroy_device)(struct libusb_device *dev);

    /* Submit a transfer. Your implementation should take the transfer,
     * morph it into whatever form your platform requires, and submit it
     * asynchronously.
     *
     * This function must not block.
     *
     * Return:
     * - 0 on success
     * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
     * - another LIBUSB_ERROR code on other failure
     */
    int (*submit_transfer)(struct usbi_transfer *itransfer);

    /* Cancel a previously submitted transfer.
     *
     * This function must not block. The transfer cancellation must complete
     * later, resulting in a call to usbi_handle_transfer_cancellation()
     * from the context of handle_events.
     */
    int (*cancel_transfer)(struct usbi_transfer *itransfer);

    /* Clear a transfer as if it has completed or cancelled, but do not
     * report any completion/cancellation to the library. You should free
     * all private data from the transfer as if you were just about to report
     * completion or cancellation.
     *
     * This function might seem a bit out of place. It is used when libusb
     * detects a disconnected device - it calls this function for all pending
     * transfers before reporting completion (with the disconnect code) to
     * the user. Maybe we can improve upon this internal interface in future.
     */
    void (*clear_transfer_priv)(struct usbi_transfer *itransfer);

    /* Handle any pending events. This involves monitoring any active
     * transfers and processing their completion or cancellation.
     *
     * The function is passed an array of pollfd structures (size nfds)
     * as a result of the poll() system call. The num_ready parameter
     * indicates the number of file descriptors that have reported events
     * (i.e. the poll() return value). This should be enough information
     * for you to determine which actions need to be taken on the currently
     * active transfers.
     *
     * For any cancelled transfers, call usbi_handle_transfer_cancellation().
     * For completed transfers, call usbi_handle_transfer_completion().
     * For control/bulk/interrupt transfers, populate the "transferred"
     * element of the appropriate usbi_transfer structure before calling the
     * above functions. For isochronous transfers, populate the status and
     * transferred fields of the iso packet descriptors of the transfer.
     *
     * This function should also be able to detect disconnection of the
     * device, reporting that situation with usbi_handle_disconnect().
     *
     * When processing an event related to a transfer, you probably want to
     * take usbi_transfer.lock to prevent races. See the documentation for
     * the usbi_transfer structure.
     *
     * Return 0 on success, or a LIBUSB_ERROR code on failure.
     */
    int (*handle_events)(struct libusb_context *ctx,
        struct pollfd *fds, POLL_NFDS_TYPE nfds, int num_ready);

    /* Get time from specified clock. At least two clocks must be implemented
       by the backend: USBI_CLOCK_REALTIME, and USBI_CLOCK_MONOTONIC.

       Description of clocks:
         USBI_CLOCK_REALTIME : clock returns time since system epoch.
         USBI_CLOCK_MONOTONIC: clock returns time since unspecified start
                                 time (usually boot).
     */
    int (*clock_gettime)(int clkid, struct timespec *tp);

#ifdef USBI_TIMERFD_AVAILABLE
    /* clock ID of the clock that should be used for timerfd */
    clockid_t (*get_timerfd_clockid)(void);
#endif

    /* Number of bytes to reserve for per-device private backend data.
     * This private data area is accessible through the "os_priv" field of
     * struct libusb_device. */
    size_t device_priv_size;

    /* Number of bytes to reserve for per-handle private backend data.
     * This private data area is accessible through the "os_priv" field of
     * struct libusb_device. */
    size_t device_handle_priv_size;

    /* Number of bytes to reserve for per-transfer private backend data.
     * This private data area is accessible by calling
     * usbi_transfer_get_os_priv() on the appropriate usbi_transfer instance.
     */
    size_t transfer_priv_size;

    /* Mumber of additional bytes for os_priv for each iso packet.
     * Can your backend use this? */
    /* FIXME: linux can't use this any more. if other OS's cannot either,
     * then remove this */
    size_t add_iso_packet_size;
};

extern const struct usbi_os_backend * const usbi_backend;

extern const struct usbi_os_backend linux_usbfs_backend;
extern const struct usbi_os_backend darwin_backend;
extern const struct usbi_os_backend openbsd_backend;
extern const struct usbi_os_backend windows_backend;

#endif

